This invention relates to the processing of fruit and vegetable juices, and more particularly, this invention relates to controlling the Brix of a concentrated juice product discharged from an evaporator.
For many years, the TASTE (thermally accelerated short-time evaporation) evaporator has provided a preheating cycle and first evaporation stage in the efficient processing of juice into a juice concentrate. This process can include pasteurization and juice stabilization. In a multi-stage, multieffect evaporator, steam is typically used on a first effect. Each subsequent effect is heated by the vapor evaporated in the preceding stage. Vapor starts at the highest pressure and ends at the stage having the lowest pressure. Any vessel where steam is flowing is known as the effect, and the area where juice flows is the stage. Evaporation takes place in one or more stages following the feed stage and the first effect, as is well known.
It is desirable if the variability of the final juice product concentration discharged from the TASTE evaporator is minimized. Accurate control over this process, however, is difficult, because of the long dead time in the process, and the constantly changing disturbance variables and coupled variables, e.g., two supposedly independent variables that are related to each other, such as (1) change in the feed Brix, and (2) an affect on the temperature.
This process dead time is difficult to overcome because in traditional feedback control methods, a change can be made only after an error is detected. If the process dead time is long, then the error can progress over an extended period of time before a correcting change is recognized. Many factors outside the immediate process can affect the evaporator control, including variations in the feed Brix, changes in steam pressure and quality from the boiler, weather conditions, and the condition of the evaporator. Because TASTE evaporators are arranged in a cross flow configuration, coupling of certain variables occur. A change in vapor pressure on the shell side of the evaporator will cause changes in juice concentration in the intermediate stages of the evaporator. Consequently, changes in feed juice concentration will cause changes in the evaporator shell-side vapor temperatures that can affect juice concentration in other stages of the evaporator before the actual feed concentration change propagates through the stages of the evaporator.
Traditional methods of TASTE evaporator control involve measuring the final product juice concentration and then adjusting the feed juice flow rate to maintain the product concentration at a desired level. This method can be accomplished either manually or automatically. In the manual method, an operator measures the final product Brix for the juice concentration, and then makes adjustments to the feed flow rate. This type of control can also be accomplished automatically with an industrial control computer and proper instrumentation. In either case, the control method is a feedback control that inherently requires a deviation from the desired final product concentration to make a change to the feed juice flow rate. The variability inherent in this type of control method can often cause the final product to be out of range from specified parameters. It should be understood that the system can use pressure or steam mass flow. Both relate to the energy fed to the evaporator.
For example, if an error is detected between the setpoint and the actual value and the change is made in one of the evaporators to the feed flow rate, which is a controlling or process variable, it may take 15 minutes before that change is made in the final concentrated juice product. This is an extended period of time that is not acceptable in all applications.
It is therefore an object of the present invention to provide a predictive model for controlling Brix in a final concentrated juice product that is discharged from a multi-stage, multi-effect evaporator, such as a TASTE evaporator.
The present is advantageous and provides a method and system of controlling the Brix of a concentrated juice product. Juice is passed through a multi-stage, multi-effect evaporator having juice and steam passing therethrough in a vacuum to form a concentrated juice product. The results of a process change in disturbance variables on the vapor temperature in at least one of the effects of the evaporator is predicted. The Brix of the concentrated juice product discharged from the evaporator is controlled based on the predicted results by controlling one of steam or juice flowing to the evaporator.
In yet another aspect of the present invention, the step of predicting the result of the disturbance variable includes the step of measuring the Brix of the juice fed into the evaporator as a disturbance variable, and measuring the pressure of juice fed into the evaporator as a disturbance variable. The pressure of steam fed into the evaporator can be controlled based on the predicted results of a change in the disturbance variables. The pressure of juice fed into the evaporator can also be changed. This step of predicting the result of changes in disturbance variables on the vapor temperature can occur in at least one of either the first or second effects of the evaporator. A steam setpoint is also established, and in one aspect of the invention, predictive models are cascaded to establish, first, a steam setpoint, and establish, second, the steam pressure into the evaporator.
In yet another aspect of the present invention, the Brix of the concentrated juice product discharged from the evaporator can be measured to form a predictive feedback model based on both the measured Brix and the process changes in the disturbance variables.